1. The URETHRA is a fibromuscular tube that carries the urine from the bladder to the exterior. The mucosa has large longitudinal folds around the lumen. The urethral epithelium has a stratified columnar nature. This thick epithelial lining varies between stratified columnar in some areas and pseudostratified columnar elsewhere, but it becomes stratified squamous at the distal end of the urethra.

In males, the urethra is longer and has 3 segments. These are:
– Prostatic urethra – extends through the prostate gland and is lined by urothelium.
– Membranous urethra – is lined by stratified columnar and pseudostratified columnar epithelium.
– Spongy urethra – is enclosed within erectile tissue of the penis and is lined by stratified columnar and pseudostratified columnar epithelium, with stratified squamous epithelium distally.

In women, the urethra is a 3-5 cm-long tube, lined initially with transitional epithelium which then transitions to nonkeratinized stratified squamous epithelium.

The middle part of the urethra in both sexes is surrounded by the external striated muscle sphincter.

2. UROTHELIUM or transitional epithelium is a specialized type of tissue that lines the inside of the urinary tract. When the bladder is empty, the mucosa is highly folded and the urothelium has bulbous umbrella cells. On the other hand, when the bladder is full, the mucosa is pulled smooth, the urothelium is thinner, and the umbrella cells are flatter.

3. The above drawing is a transversely cut MEDULLARY RENAL PYRAMID. The pyramid consists mainly of tubules that transport urine. It shows closely packed cross-sections of the many nephron loops’ thin descending and ascending limbs and thick ascending limbs, intermingled with parallel vasa recta capillaries containing blood and collecting ducts.